- cut the belt (easy) - get rid of the idler (allen screw) - dispose of the aircon belt - put the eye of 15mm combined spanner on the nut behind the tensioner and then it was no problem tightening the 24mm bolt (anti-clockwise) Little setback: I ordered a PQS101310 to replace the aircon belt...

Same problem: tensioner 24 mm bolt turning not releasing the belt. After trying to place the 15mm spanner behind the tensioner I realized I wouldn't succeed. So did the following: